While cured epoxy resin systems based on bisphenol A and other aromatic polyols generally exhibit good mechanical properties, they show a relatively poor resistance to weathering in outdoor applications or upon exposure to ultraviolet light.
Since such weathering problems are clearly understood to be related to the aromatic moieties in the bisphenol A epoxy resins, epoxy resins having no aromatic moieties have been made. Lee and Neville, Handbook of Epoxy Resins, McGraw-Hill, New York, 1967, p 2-16 describes such epoxy resins derived from aliphatic polyols.
High molecular weight polyglycidyl ethers, prepared by the hydroformylation of oleyl alcohol and related unsaturated alkenols followed by glycidylation of the polyol formed, are described in U.S. Pat. Nos. 4,388,209 and 4,339,389. Such glycidyl ethers are stated to be curable by a host of hardener types, i.e. amines, anhydrides, acids, polyaminoamides, melamines or mercaptans, with only amine curing being exemplified.
While aliphatic epoxy resins are expected to exhibit better weather-resistance than aromatic epoxy resins, they are usually deficient in such properties as a combination of hardness and flexibility (toughness) and in chemical, solvent and moisture resistance as is taught in U.S. Pat. No. 4,540,752.
U.S. Pat. 4,528,345 describes cycloaliphatic resins prepared by the hydrogenation of the glycidyl ethers of bisphenol A, but points out that in order to cure such resins with polyamide hardeners an induction or prereaction period of about one hour is essential lest the cured coating suffer a rapid loss of gloss and surface defects such as blushing and blemishing.
Aliphatic glycidyl ethers have been used in cold-curable, solvent-free coating compositions using cycloaliphatic polyamines as curing agents to give cured coatings with high resistance to organic solvents as described by J. Bersier et al, FATIPEC, 1970, 255-276. Similar coating compositions were tested for color and gloss retention in outdoor weathering applications. Such coatings had excellent resistance to discoloration, but showed a high loss of gloss after only a few months of outdoor exposure as is described by T. Audykowski et al, FATIPEC, 1974, 109-116.
Audykowski et al also teach that such aliphatic glycidyl ethers may be cured at elevated temperatures (180.degree. C.) with anhydride hardeners to give coatings with somewhat better retention of gloss after outdoor exposure than the coatings cured at room temperature with amine hardeners.
R. S. Bauer, FATIPEC, 1980, 102-122, describes pigmented coatings prepared from the diglycidyl ether of hydrogenated bisphenol A (EPONEX DRH-151.2, Shell) cured with a hydrogenated polyaminoamide (VERSAMID 1540, Henkel) or a conventional polyamide (UNI-REZ 2188, Union Camp). These coatings exhibited good retention of gloss after exposure to weathering (Weatherometer and Quick Ultra-violet, QUV testing).
Polyamidoamine hardeners are prepared by dimerizing tall oil fatty acids and then reacting the dimerized acid with aliphatic amines such as diethylenetriamine. These hardeners are described by V. Brytus, Modern Paint and Coatings, 74 (10), 172 (1984). These hardeners are alternatively known as polyamides, as polyaminoamides or as polyamidoamines.
United States Pat. Nos. 2,994,673; 3,956,208; 3,647,728 and 3,085,075 and British Pat. No. 988,484 all teach curable epoxy resin compositions containing an aliphatic polyglycidyl ether of an alkane polyol and a polyamidoamine hardener which can be cured at ambient temperatures.
Only U.S. Pat. No. 2,994,673 and British No. 988,484 mention sorbitol as one possible polyol among a large list of such polyols which might be used to prepare the aliphatic polyglycidyl ether resins. Neither reference exemplifies the use of a polyglycidyl ether of sorbitol and neither reference points out the particularly beneficial effects obtained when a polyglycidyl ether of sorbitol is cured with a polyamidoamine hardener as a method of preparing a protective coating which becomes through-cured in 2 to 24 hours at 5.degree.-80.degree. C.